Switch constraints proposal to use branch-git-prep
Switch constraints proposal to use branch-git-prep

We don't have zuul-cloner installed on our proposal worker, so use
the existing branch-git-prep macro for constraints proposal jobs
instead.

Change-Id: I877afb6816dc24436033b9af16c8e3b6c861dc0d

anbangr/osci-project-config,dongwenjuan/project-config,dongwenjuan/project-config,Tesora/tesora-project-config,anbangr/osci-project-config,openstack-infra/project-config,noorul/os-project-config,coolsvap/project-config,noorul/os-project-config,openstack-infra/project-config,coolsvap/project-config,Tesora/tesora-project-config